1. Field of the Invention
The present invention relates to a composite transformer (a combined type transformer), and more particularly to a composite transformer (a combined type transformer) which is downsized and causes a small magnetic energy loss.
2. Description of the Related Art
Conventionally, in the field of the electric-power conversion circuits such as DC-DC converters, various inventions have been made. (DC stands for direct current.)
For example, Japanese Patent Laid-open No. 2005-224058 discloses a DC-DC converter using a magnetic-field canceling transformer in which a plurality of windings are arranged so that the magnetic fluxes produced by the plurality of windings cancel each other. (Hereinafter, the magnetic-field canceling transformer may be simply referred to a transformer.) In addition, Japanese Patent Laid-open No. 2009-284647 (which is hereinafter referred to as JP2009-284647A) discloses as an improvement of the above magnetic-field canceling transformer a composite transformer in which a transformer and a buck-boost inductor are structurally integrated by using windings for the transformer and the buck-boost inductor in common.
However, in a magnetic-field canceling transformer according to a conventional technique, two windings are alternately wound around a central core leg as a portion of the transformer core. (See the central core leg 61a in FIGS. 2, 3, and 4 in JP2009-284647A.) Therefore, in the case where the wires of the two windings wound around the central core leg are also respectively wound around the inductor cores arranged on both sides of the central core leg, the windings protrude from the central core leg to both inductor-core sides of the central core leg. Further, if an attempt is made to arrange more than two windings around the central core leg, many physical constraints limit the layout, so that it is difficult to increase the number of windings to be arranged in parallel.
In addition, in the conventional composite transformer, the magnetic flux density in the central core leg around which the two windings are wound becomes so high as to exceed the saturation magnetic flux density of the transformer core, so that magnetic energy loss occurs.
Further, although the magnetic-field canceling transformer according to the conventional technique is downsized because of the common use of the windings as both of the inductor coils and the transformer coils, the conventional composite transformer is required to be further reduced in size because smaller composite transformers are more desirable.
The present invention has been developed in view of the above circumstances. The object of the present invention is to provide a composite transformer in which a plurality of windings are arranged in parallel, and the magnetic energy loss and the size can be reduced.